The technology of amplifying optical transmission systems with many channel wavelengths is rapidly progressing. Current systems typically carry 8 channels on an optical fiber, and future systems may have as many as 64 channels. The wavelengths must be very closely spaced so that all channels can be optically amplified together. A typical channel separation is 100 GHz or about 0.8 nm. The current technology has focused on combining these wavelengths (multiplexing) on a single fiber at the input of a chain of optically amplifying repeaters and separating these wavelengths (demultiplexing) at the end of the repeater chain. Future systems will also need provisions for partially altering the traffic at each repeater by adding-dropping one or several channels out of the total number. This is a challenging problem because it is desirable that the add-drop multiplexer be reconfigurable: the addition or dropping of a channel should be made without disrupting the traffic on the other channels.
There are several additional concerns. The reconfigurable add-drop multiplexer (R-ADM) should not act as a narrow band filter for the passed channels, since concatenation of such filters at many repeaters would excessively narrow the channel pass bands. The R-ADM should have low transmission loss and low cost, since these important factors ultimately determine which technology is selected. Ideally, the R-ADM should be able to add-drop more than one channel, and in some instances nearly all the channels.
One approach to a R-ADM using existing multiplexer technology is to separate all channels on different waveguides, to run each waveguide through a 2.times.2 optical switch for adding or dropping the signal and to recombine the channels in a multiplexer (See W.D. Zhong et al., "Reconfigurable multichannel add-drop multiplexer" . . . , Electronic Let., Vol. 16, No. 16, pp. 1477-78 (1996)). The problem with this approach is that the multiplexer acts as a narrow-band filter for every channel. Successive application of such multiplexers has the drawback of narrowing the channel pass bands.
Another approach uses an optical fiber containing a series of Bragg reflectors with reflection bands residing in "guard bands" between channels (See L. Quetel et al., OFC '96 Technical Digest, xxx, pp. 120-121). To drop a channel, the part of the fiber containing a Bragg reflector is heated or stretched, shifting the Bragg reflection band out of the guard band and causing Bragg reflection of a particular channel. This approach has the advantage that all the channels can be dropped, but if more than one channel is add-dropped, a combiner and a multiplexer are needed to combine the added channels and to separate dropped channels. Optical circulators are needed at the ends of the optical fiber to separate the add and drop ports from the input and output of the passed channels, without incurring excessive insertion loss. But the method reserves about half the spectrum for storing the Bragg reflection bands between channels. Accordingly it is not attractive as the number of channels to be squeezed into the Er amplification band increases. Accordingly, there is a need for an improved reconfigurable add-drop multiplexer.